1. Field of the Invention
The present invention relates to a wavelength allocation method of signal light on wavelength grid in a wavelength division multiplexing optical transmission, and in particular, relates to a wavelength allocation method for signal light, which can suppress transmission characteristic degradation caused by four-wave mixed light generated in an optical transmission path, and an optical transmission apparatus and a wavelength division multiplexing optical transmission system using the method.
2. Description of the Related Art
Due to the tremendous increase in IP traffic, the demand for large capacity and low cost optical transmission systems has increased rapidly. To meet this requirement, one solution is to further increase the capacity and lower the cost of wavelength division multiplexing (WDM) optical transmission systems. Dense wavelength division multiplexing, in which spacing between wavelengths is reduced, is considered as means for increasing WDM capacity.
FIG. 12 shows an example of a typical WDM optical transmission system.
The WDM optical transmission system shown in FIG. 12 comprises, for example, an optical transmission terminal apparatus 1, optical repeater apparatuses 2, an optical reception terminal apparatus 3, and an optical transmission path 4. The optical transmission terminal apparatus 1 comprise: optical senders (OS) 1A for each wavelength; variable optical attenuators (VOA) 1B adjusting the levels of signal lights output from the optical senders 1A, for each wavelength; an optical multiplexer (MUX) 1C multiplexing the signal lights of respective wavelengths to output a WDM signal light, and a post-amplifier 1D amplifying the WDM signal light directly in optical to send the amplified signal to the optical transmission path 4. A part of the WDM signal light output from the post-amplifier 1D may be sent to an optical spectrum analyzer (OSA) 1E, and a transmission state thereof may be monitored.
The optical reception terminal apparatus 3 comprises: a preamplifier 3A receiving and amplifying the WDM signal light repeatedly transmitted from the optical transmission terminal apparatus 1 via the optical transmission path 4 and the optical repeating apparatuses 2; an optical demultiplexer (DEMUX) 3B demultiplexing the WDM signal light amplified by the preamplifier 3A into signal lights of respective wavelengths; and optical receivers 3C receiving the signal lights. A part of the WDM signal light received by the preamplifier 3A may by sent to an optical spectrum analyzer (OSA) 3D, and a reception state thereof may be monitored.
As the optical repeater apparatus 2, there is for example an optical amplification-repeating apparatus comprising only an optical amplifier for amplifying the WDM signal light having been attenuated in the optical transmission path 4. As another example of the optical repeater apparatus 2, there is an optical repeater apparatus which operates not only as an optical amplifier, but also as an OADM (Optical Add/Drop Multiplexing) node having an optical add/drop multiplexing function, or as an optical compensation node having functions for compensating for accumulation of optical level deviations between wavelengths, which is a particular problem in the transmission over long distance, and for compensating for wavelength dependence of cumulative dispersion attributable to a difference in dispersion slope rate between the transmission path and a dispersion compensator compensating for wavelength dispersion in the transmission path. For the OADM nodes or optical compensation nodes, there have been known a method in which the optical add/drop multiplexing, the optical level deviation compensation, and the dispersion slope compensation are performed for each individual wavelength, or a method in which signal lights of a plurality of wavelengths are grouped into a single wavelength group to form a plurality of wavelength groups, and the optical add/drop multiplexing, the optical level deviation compensation and the dispersion slope compensation are performed for each of the respective wavelength groups.
FIG. 13 shows an example of wavelength allocation of signal lights, in the case where the aforementioned wavelength groups are applied.
In the wavelength allocation shown in FIG. 13, a band of 8 wavelengths comprising a signal band in which signal lights of six consecutive waves are present, and a wavelength protection band for two wavelengths, in which signal lights are not present, is set as a single wavelength group. Here, such a wavelength group comprising signal lights of q consecutive waves and a wavelength protection band for r wavelengths, is called a (q, r) wavelength group. The example In FIG. 13 shows (6, 2) wavelength groups. In such a (q, r) wavelength group, due to the presence of the wavelength protection band, the wavelength spacing becomes wider in some parts of the overall signal band. Therefore, there is an effect that an influence of non-linear effect occurring in the optical transmission path is reduced. Here, the wavelength protection band has a value depending on a transmission characteristic of a multiplexer/demultiplexer used in the OADM node or the optical compensation node.
In such a conventional WDM optical transmission system, by using about 0.4 nm wavelength spacing (50 GHz spacing), and both the 1550 nm band (C-band) and the 1580 nm band (L-band) as signal wavelength bands, the transmission in 160 or more or wavelength multiplexing and 1.6 Tb/s or more of transmission capacity, is realized. Moreover, in order to cope with the expected further rapid increase in traffic, development or super dense multiplexing for increasing the signal light wavelength band and further reducing the width of the signal light wavelength spacing is anticipated.
To be specific, assuming the case where the 50 GHz spacing currently implemented as the wavelength spacing between signal lights is halved to 25 GHz spacing, a problem occurs in that waveform distortion due to the non-linear effect in an optical fiber is increased, which leads to marked degradation in the transmission characteristic. In particular, when using, as the optical transmission path, an optical fiber in which a wavelength dispersion value in the signal wavelength band is in the vicinity of 0 to ±10 ps/nm/km, the coherent crosstalk attributable to four-wave mixed light generated by four-wave mixing, being one of the non-linear effects which occurs in the optical transmission path, becomes a cause of marked degradation in the transmission characteristic of the WDM signal light.
Conventionally, various techniques for suppressing this coherent crosstalk attributable to four-wave mixed light have been proposed, such as a method in which the wavelength allocation of signal lights is shifted slightly from the ITU grid, to realize unequally spaced allocation (the following literature 1), a method which uses the wavelength groups as described above (the following literatures 2 and 3), and a method in which the polarization of the signal lights of adjacent wavelengths are made orthogonal to each other and multiplexed (the following literature 4). The ITU grid is the wavelength allocation of signal lights, standardized by the international Telecommunication Union (ITU).
(Literature 1)
F. Forghieri, R. W. Tkach, A. R. Chraplyvy, and D. Marouse, “Reduction of four-wave mixing crosstalk in WDM systems using unequally spaced channels,” IEEE Photon. Technol. Lett., vol. 6, pp. 754-756, June 1994.
(Literature 2)
I. Haxell, M, Ding, A. Akhtar, H. Wang, and P. Farrugia, “52×12.3 Gbit/s DWDM transmission over 3600 km of True Wave fiber with 100 km amplifier spans,” PD5,OAA 2000.
(Literature 3)
Xiang-Dong Cao and Tau Yu, “Ultra long-haul DWDM transmission via nonlinearity management,” OtuC5, pp.140-142,OAA 1999.
(Literature 4)
Neal S. Bergano et al., “320 Gb/s WDM Transmission (64×5 Gb/s) over 7,200 km using Large Mode Fiber Spans and Chirped Return-to-Zero Signals,” OFC'98, postdeadline papers PD12, San Jose. USA, February 1998.
However, this conventional technology presents the following problems. Namely, with the method in which the unequally spaced allocation is used in the wavelength allocation of signal lights, since the wavelength allocation is shifted from the ITU grid, optical multiplexers/demultiplexers corresponding to ITU grid cannot be used, and the structures of optical multiplexers/demultiplexers are more complicated, which increase the cost of the system. Furthermore, with the method in which the wavelength groups are used, if (q, r) wavelength groups are used over the entire signal band, the frequency of signal light wavelength multiplexing is reduced by q/(q+r) times than that when signals are allocated in each individual wavelength, and this leads to the reduction in transmission capacity. In addition, with the method in which the polarization of the signal lights of adjacent wavelengths are made orthogonal to each other and multiplexed, it is necessary to keep a polarization state of the signal light constant until the signal light of each wavelength output from the optical transmitter is multiplexed in the optical multiplexer to be output as WDM signal light, which results in a costly system.